vim: refactor database handling

To minimize memory usage we need to destroy the queries and the
databases, so we should keep track of them.

Each buffer gets a database connection that is destroyed when the buffer
is destroyed, and all the queries along with it.

Ideally notmuch should destroy the queries when the database is
destroyed, but it's not doing that at the moment.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>